The present disclosure relates to a case hardening method for high performance long life martensitic stainless steel bearings.
Modern aerospace bearings are required to sustain and transfer high stresses from radial and/or axial loads under rapid rotational motion, over a range of temperatures from sub-zero to those that challenge the capability of the lubricant and steel materials. As such, the steel material has to have high load bearing capability (strength and hardness) and other characteristics that include, but are not limited to, temperature capability, fracture toughness, wear resistance, and oil-out capability. One key material characteristic that is embodied in those fundamental requirements is the rolling contact fatigue (RCF) endurance limit of the steel. This characteristic is used in the design and lifting of mechanical systems components (bearings and gears).
The RCF endurance limit of the bearing steels is defined as the materials capability limit when a surface spall initiation event occurs. That event is caused by a combination of independent bearing operating variables, stress, temperature and rolling contact fatigue cycles/time.
Martensitic steels that are suited for over 95% of high performance bearings and gears due to their unique balance of physical and mechanical properties: moderately high elastic modulus, thermal conductivity, ultra-high hardness and compressive strength, limited to moderate fracture toughness, temperature capability to approximately 600F, and rolling contact fatigue resistance.
Current hardening processes for martensitic stainless steels take over 100 hours of carburization time in high cost furnaces like vacuum carburization or plasma carburization. Such process times are more than two times required for conventional case hardening non-stainless martensitic steels, and at proportionally higher cost. The cost negatively impacts the cost of component design to cost metrics.
There is a need for a method which significantly reduces case hardening time for high performance long life martensitic stainless steel bearings.